This invention relates to a process for use in underground retorting of oil shale, and more particularly, to a process for determining retorting conditions in an in situ oil shale retort.
Researchers have now renewed their efforts to find alternative sources of energy and hydrocarbons in view of recent rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material, such as oil shale, coal and tar sands by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily useable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil is contained in oil shale deposits in the United States with almost 60 percent located in the rich Green River oil shale deposits of Colorado, Utah and Wyoming. The remainder is contained in the linear Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis for these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as oil shale retort water, hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
The process of pyrolyzing the kerogen and oil shale, known as retorting, to form liberated hydrocarbons can be done in in situ retorts under ground or in surface retorts above ground. In principle, the retorting of oil shale comprises heating the oil shale to an elevated temperature and recovering the vapors and liberated effluent. However, as medium grade oil shale yields approximately 20 to 25 gallons of oil per ton of shale, the efficiency of retorting is critical to the economic feasibility of a commercial opera- tion.
In in situ retorts, a flame front is continuously or intermittently passed through a bed of rubblized oil shale to liberate shale oil, off gases and oil shale retort water. There are two types of in situ retorts: true in situ retorts and modified in situ retorts. In true in situ retorts, all of the oil shale is retorted under ground as is, without mining or transporting any of the shale to aboveground locations. The shale can be explosively rubblized, if desired. In modified in situ retorts, some of the oil shale is mined and conveyed to the surface to create a cavity or a void space in the retorting area. The remaining underground oil shale above the void is then explosively rubblized to substantially fill the void. The oil shale which has been removed is conveyed to the surface and retorted above ground.
Over the years various methods have been suggested for detecting the location of the flame front and other retorting conditions in an in situ oil shale retort. Typifying these methods are those found in U.S. Pat. Nos. 4,082,145; 4,120,354; 4,148,529; 4,149,592; 4,150,722; 4,151,877; 4,162,706; 4,163,475; 4,166,721; 4,199,026; 4,223,726; 4,227,574; 4,249,602; 4,249,603; 4,252,374; 4,263,969; and 4,279,302. These prior art methods have met with varying degrees of success.
It is therefore desirable to provide an improved method or process for determining retorting conditions in an in situ retort.